The key elements of a gravity gradiometer are a pair of substantially identical spaced masses and the object is to measure differences between the gravitational force on the respective masses. Effectiveness requires measurements of this difference when it approaches only one part in 10.sup.12 of normal gravity. Approaches to measuring gravity gradients have thus far fallen into two broad classes. The first of these entails differential modulation of a signal or parameter by the difference between the gravitationally induced accelerations of the two masses. The second technique involves direct measurement of the net gravitational acceleration of one mass relative to the other.
British patent publication 2022243 by Standard Oil Company discloses a gravity gradiometer in the first class. An element, described in the patent publication as a mass dipole dipoles as described, is mounted coaxially on one end of a photoelastic modulator element positioned in the cavity of a ring laser tube to differentially modulate circular polarization modes in response to application of a torque. In a preferred form, two mass dipoles as described are mounted on opposite ends of the modulator element to balance rotational acceleration noise. A closely related development by the same inventor, Lautzenhiser, described in U.S. Pat. No. 4,255,969, employs actual mass dipoles in conjunction with respective photoelastic modulator elements.
Another modulation technique involves rotating a platform which is supporting suitable arrangements of mass pairs. Various instruments of this kind are summarised by Jekeli at 69 EOS (No. 8). One of these, by Metzget, has been further developed and consists of electronically matched pairs of accelerometers on a rotating platform. The platform modulates the sum of opposing acceleration signals with a frequency twice its rotational frequency. These modulation systems call for extremely exacting uniformity in the rotation and require the use of bearing, rotational drive and monitoring technology which is not yet of a standard to render the instruments practicably suitable on an appropriate scale for airborne or moving land-based measurements for geophysical resource exploration, as opposed to geodetic surveying. The alternative of directly measuring gravity gradient components necessitates a weary high degree of electronic, magnetic, thermal and vibration isolation to achieve the measurement accuracy needed. Machines thus far have had poor spatial resolution and a high noise level.
An instrument for measuring the diagonal components g.sub.xx, g.sub.yy and g.sub.zz of the gravitational gradient tensor is described by van Kann et al in the publication IEEE Trans. Magn. MAG--21, 610 (1985) and further elaborated in the NERDDP End-Of-Grant Report (1986) on project no. 738. This instrument consists of a pair of accelerometers mounted with their sensitive axes in line. The difference in displacement of the accelerometers is proportional to the component of the given tensor gradient and is sensed by the modulated inductance of a proximate superconducting coil. This instrument suffers from the disadvantage that diaphragm springs serve both as mounts for the masses and as gradient sensors. The former of these roles calls for a greater stiffness in the springs while the sensing role necessitates enhancement of the springs' softness. It is also very difficult with the van Kann instrument to achieve axial alignment of the masses and trimming of the spring mountings with the accuracy needed to obtain the common mode acceleration rejection ratios necessary for the accuracy sought.